The intervertebral disc is a complex joint anatomically and functionally. It is composed of three component structures: the nucleus pulposus, the annulus fibrosus and the vertebral endplates. The biomedical composition and anatomical arrangements within these component structures are related to the biomechanical function of the disc.
The nucleus pulposus occupies 25-40 percent of the total disc cross-sectional area. It is composed mainly of mucoid material containing mainly proteoglycans with a small amount of collagen. Due to these constituents, the nucleus pulposus has the capacity to bind water and usually contains 70-90 percent water by weight. Because of this high water content, the nucleus may be mechanically described as an incompressible hydrostatic material. The disc is under constant compressive forces even when the spine is not weight bearing as a result of the tension applied by the annulus fibrosus and the intervertebral ligaments.
The annulus fibrosus is a concentrically laminated structure which contains highly aligned collagen fibers and fibrocartilage embedded in amorphous ground substance. The annular layers are oriented at .+-.30 degrees to the longitudinal axis of the spine. In the inner laminae, these annular layers are anchored to the cartilainous endplate while the outermost layer is attached directly into the osseous tissue of the vertebral body. Usually, the annulus fibrosus has approximately 8-12 layers and has an anterior portion which is about 1.2-1.5 times thicker than its posterior region. Mechanically, the annulus fibrosus is the main stabilizing structure which resists torsional and bending forces applied to the disc. A normal isolated disc provides approximately 35 percent of the torsional rigidity of a whole intervertebral joint.
The two vertebral endplates are composed of hyaline cartilage and separates the disc from the adjacent vertebral bodies. This layer acts as a transitional zone between the hard, bony vertebral bodies and the softer disc.
The spinal disc may be displaced or damaged due to trauma or a disease process. If this occurs, the nucleus pulposus may herniate and protrude into the vertebral canal or intervertebral foramen, in which case, it is known as a herniated or "slipped" disc. This disc may in turn press upon the spinal nerve, that exits the vertebral canal through the partially obstructed foramen, causing pain or paralysis in the area of its distribution. The most frequent site of occurrence of a herniated disc is in the lower lumbar region. A disc herniation in this area often involves the inferior extremities by compressing the sciatic nerve. To alleviate this condition, it may be necessary to remove the involved disc surgically and fuse the two adjacent vertebrae. A number of procedures have been identified and are described in the orthopaedic literature. One such is described in "Orthopedics-Principles and Their Application", Samuel L. Turek, M. D., Lippincott Company, Third Edition, pp. 761-763. In this procedure, a hole is drilled in the spinal column straddling the damaged disc space and the two adjacent vertebral bodies. The hole is then filled with a cylindrical plug or dowel in order to fuse the vertebrae together. The fusion procedure is an excellent method of eliminating symptoms and yet maintaining joint stability, but at the expense of total loss of motion of the fused vertebral joint. The adjacent discs will have increased motion and stress due to the increased majority of the fused segment. In the long term, this change in mechanics of the motion of the spine causes these adjacent discs to degenerate. Obviously, a more desirable situation would involve replacing the damaged disc with a suitable biofunctional equivalent so as to return the patient's spine to normalcy. Heretofore, the development of a prosthetic joint device to replace the injured intervertebral disc has been unsuccessful du to the complexity of the structure and biomechanics of the normal disc.
Other spacers for spinal repair have been developed, see for instance those of U.S. Pat. No. 3,867,728, U.S. Pat. No. 4,309,777, U.S. Pat. No. 4,349,921 and U.S. Pat. No. 4,553,273. None of these, however, have been commercially developed. U.S. Pat. Nos. 4,349,921 and 4,553,273 are essentially rigid bodies which serve to stabilize the spine but do not allow motion within the disc itself. U.S. Pat. No. 4,309,777 consists of a disc which allows motion, but this is achieved by the use of springs contained within the body of the disc.
U.S. Pat. No. 3,867,728 by Stubstad et al. discloses a device which replaces the natural disc with one of similar shape and strength. The disc may be constructed from an elastic polymer such as silicone and reinforced with fabric. The top and bottom surfaces may be provided with an open pored material such as a velour to encourage tissue in growth. The purpose of this invention is to provide a system capable of withstanding the loads imposed upon it during normal human activities. As a result, the preferred construction of the disc provides for reinforcement against only compressional loads. In practice, the spine is subjected to both compressional and torsional loading and, to be successful, any device must be capable of withstanding both forms. In addition to strength, any prosthetic disc must deform elastically in a similar manner to the natural structure in order that normal stresses are induced within the adjacent vertebral bodies. If too stiff a structure is used, then the disc will deform too little, and the natural discs both superior and inferior to the prosthetic device will be required to deform excessively. This is a similar situation to that which occurs when bony fusion across the disc is employed. If, on the other hand, the device possesses too little stiffness, either in compression or torsion, then excessive motion will occur, the device will bulge out and pain may result. This is an equivalent situation to a failed bony fusion. U.S. Pat. No. 3,867,728 describes a device which is concerned only with the ultimate strength and not with any elastic properties. Therefore, the reinforcement of the elastomer through a fabric layer results only in an increase in compressional strength and fails to address the equally important problem of elasticity in compression and torsion.